The present invention relates to the field of ion exchange membranes. Ion exchange membranes contain functional groups bearing positive and/or negative ionic charges fixed to a matrix. The synthesis, properties and uses of such membranes has been reviewed in Synthetic Polymeric Membranes by R. E. Kesting, McGraw-Hill Book Company, New York (1971) and more recently by T. Sata in Pure & Appl. Chem.,58, 1613 (1986). The ionic nature of these materials make them hydrophilic and they find use in a variety of separation processes in which water is one of the principle components, including electrodialytic processes such as electrodialytic concentration and separation processes, electrodialytic water splitting, electrolysis or electrolytic splitting of water, fuel cells for electricity generation and pressure or chemical potential driven membrane processes such as ultrafiltration, reverse osmosis, piezodialysis, diffusion dialysis and pervaporation. For example, U.S. Pat. No. 4,012,324 describes the use of ion exchange membranes for use in ultrafiltration and points out the advantages of these membranes with respect to fouling resistance. Ion exchange membranes of the charge mosaic type are required for piezodialysis, a pressure driven system capable of separating salts from uncharged materials. Ion exchange membranes have also been used for drying of gases and liquids.
The configuration of equipment used for various membrane processes also varies widely. Electrodialytic processes are almost exclusively carded out with flat sheets of membranes arrayed between planar electrodes. The pressure driven processes are more likely to be carded out in a more compact configuration of either spiral wound or hollow fiber units.
While each application of ion exchange membranes has some specific requirements and one property or another may be more important depending on the application and the type of equipment in which the membranes are mounted, there are some properties which are generally desirable. Chemical stability, not only to the normal process streams but also to possible cleaning agents is essential. Mechanical strength and resistance to compaction are desirable. Ease of membrane formation and control of properties are also highly desirable.
Ion exchange membranes have been produced via graft polymerization. Early examples of this type of membrane synthesis involved thermal methods, usually in the presence of crosslinking agents to insure adherence of the graft to the backbone. More recently, radiation grafting of films with ionic monomers or their precursors has been studied more extensively and has been used commercially to produce ion exchange membranes by RAI Research Corporation, Hauppauge, N.Y. and Morgane (Courbevoie Haut de Seine, France). While membranes made via these grafting processes achieve a high degree of selectivity and other desirable transport properties, the properties are difficult to control because of the heterogeneous nature of the reactions used for grafting and the dependence of the membrane's final properties on the morphology of the starting film, which is also very difficult to control. Moreover, preparation of asymmetric structures which are highly desirable for the pressure driven processes is also difficult because of the insolubility of the resulting graft copolymers.
Various monomers and electrophiles have been grafted onto backbone polymers via lithiation followed by anionic grafting. For example, the formation of graft copolymers of poly(2,6-dimethyl- 1,4-phenylene ether) (PPE) and isoprene, methylmethacrylate, hexamethylcyclotrisiloxane or phenyl isocyanate via lithiation and subsequent anionic graft polymerization have been disclosed by Chalk and Hoogeboom, Anionic Graft Polymerization of Lithiated Poly(2,6-dimethyl-1,4-phenylene Ether), J. Poly. Sci.: Part A-1, vol. 7, 2537-2545 (1969).
The addition of a variety of non-monomeric electrophiles to polysulfone via lithiation has been disclosed in Novel Polysulfones for Membrane Applications, M. D. Guiver, OKutowy, W. A. McCurdy, I. W. Simpson, Proc. of the Int. Membrane Conf. on the 251th Anniv. of Memb. Res. in Canada, Ottawa, September 1986, 187-202 (NRC Publication No. 26413). U.S. Pat. Nos. 4,797,457 and 4,833,219 disclose substituted polysulfones and a process for prepaxing an aromatic polysulfone via metalating the polysulfone with a metalating agent; and quenching the metelated product with an electrophile so as to replace the metal substitution by an aliphatic or aromatic substituent, a hetero-atom or hetero-atom-containing group, another metal or metal containing group.